Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already declare readOnlyHint=true and openWorldHint=false, so safety is covered. The description adds genuinely useful semantics beyond that: means are computed per methodology version and never across a methodology change, which is non-obvious behavior an agent must respect when interpreting results.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

Three tight sentences with the core resource stated first, then usage, then a critical caveat. No filler, though the third sentence's phrasing ('never across a change in how the number is built') is slightly abstract and could be plainer.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

With no output schema, the description partially compensates by indicating the return shape (daily fixings, methodology-scoped means). For a two-parameter read-only history tool with full schema coverage and safety annotations, the picture is close to complete, though it says nothing about volume or pagination for a series that can span back to a rate's inception.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100% and the enum of indices is fully documented in the schema, including the BTC-COLLATERAL variants and their effective date. The description adds no parameter-level detail (e.g., what 'days' defaults to or interacts with), so the baseline 3 applies.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

The description states a concrete resource and scope: 'Daily fixings since each rate began,' which reads as a time-series of historical fixings distinct from a single-rate lookup. It does not explicitly name the sibling get_rate or clarify boundary cases, so it stops short of full sibling differentiation.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

It gives a clear when-to-use signal ('to see whether a rate is unusual, or how the cost of capital has moved'), which tells the agent the analytical intent behind the call. However, it names no alternatives (get_rate, compare_rate) and offers no exclusions, so the agent must infer when not to use it.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.